Long term exposure to low levels of arsenic is associated with multiple diseases including skin, lung, liver, and prostate cancers, as well as metabolic disorders including hypertension and diabetes. Arsenic disrupts the regulation of genes activated by steroid hormones including those regulated by the glucocorticoid receptor (GR) that can underlie many of the same diseases as arsenic. Glucocorticoids have a role in mediating immunological and stress responses, glucose homeostasis, and cell growth and differentiation. These responses are regulated by GR-mediated gene transcription. Genes are organized in chromatin and post-translational histone modification contributes to changes in chromatin structure leading to transcriptional activation or repression. The affect of arsenic on transcription may be through disruption of cell signaling pathways that impinge on the histone acetylases, deacetylases and methylases that regulate histone modification. There is a clear link between cancer and functional disruption of these proteins with promoters. Our hypothesis is that arsenic disrupts gene expression from steroid hormone regulated promoters by changing patterns of histone modification. GR-mediated histone modifications, acetylation and methylation, will be identified at the mouse mammary tumor virus (MMTV) promoter in the presence and absence of sodium arsenite in vivo by chromatin immunoprecipitation (ChIP) analysis (Specific Aim I). Arsenic effects on the recruitment or activity of proteins responsible for the modifications will be evaluated in cells in vivo by ChIP analysis and on assembled chromatin templates in vitro by western blot analysis. Arsenic-mediated protein modifications will be identified with radio-labeled cofactors by western blot analysis (Specific Aim II). To make a functional link between histone modifications in Specific Aim I and associated proteins in Specific Aim II with transcriptional activation, the identified proteins will be tested in cells in vivo in RNA interference (RNAi) analyses and in vitro enzyme activity assays (Specific Aim III).